Optical communication systems are a fast-growing constituent of communication networks. The term “optical communication system” as used in the following relates to any system or device which makes use of optical signals to transport information across an optical waveguiding medium. Optical communication systems comprise inter alia telecommunication systems, local area networks (LAN), cable television systems etc.
Wavelength division multiplexing (WDM) is a technique which allows to increase the capacity of fibre optic networks. In a WDM system, a plurality of optical channels are carried over a single waveguide, each channel being assigned a particular wavelength. The use of WDM systems for signal transmission is described in U.S. Pat. No. 5,959,750, the disclosure of which is incorporated herein by reference.
For the transmission capacity of optical fibres in optical communication systems is expected to advance in the future, the evolution of optical amplification is one of the core technologies involved in this process. A key to this evolution is the availability of extremely-broad-band optical amplifiers, offering amplification over nearly all the transmission window allowed by silica. These requirements can be met inter alia by Raman amplification.
Optical fibre Raman amplifiers (FRA) are well known and are known to be designed to operate at a desired wavelength between 1.25 μm and 1.7 μm. FRA utilize silica-based fibres and display a high transparency when unpumped. The working principle of FRA is based on stimulated Raman scattering as for example explained in the Ph.D. thesis of P. Riedel with the title “Untersuchungen zum künftigen Einsatz solitonengestützter faseropticher Nachrichtenübertragung bei 1.3 μm Wellenlänge”, Hamburg 1998, the disclosure of which is incorporated herein by reference. FRA can serve for example as a replacement for conventional repeaters or semiconductor-amplifiers, or for rare-earth-doped fibre amplifiers or in combination with them.
Y. Emori and S. Namiki disclose in OFC (Optical Fibre Communications Conference) 99, PD19-1 to PD19-3 Raman amplifiers which are pumped and gain equalized by 12-wavelength-channel WDM high power laser diodes. Every laser has to be multiplexed by a 11-MZI-planar lightwave circuit (PLC) and individually power-supplied by means of a controlling unit.
When Raman amplification in optical communication systems is used as mentioned above, i.e. in combination with several WDM wavelength pump channels, interference between adjacent pump channels will occur, i.e. they “crosstalk”. This is due to stimulated Raman scattering and four-wave-mixing. Therefore, significant pumping efficiency will be lost in addition to pump instabilities caused by polarisation of the interfering channels.
Several solutions for these problems have been proposed:
The use of a Raman fibre laser as pumping means provides a high pump efficiency output which is located in only one channel. The use of two or more Raman fibre lasers is possible but is more costly and power-consuming. Raman lasers are not polarized.
Alternatively, the use of two semiconductor pump lasers for each required pump wavelength combined with a PBC (polarisation beam combiner) has been proposed, which allows to obtain an unpolarized output. However, this leads to a loss of power at the pump output in the range of 0.5 dB to 1 dB and requires an additional step in the fabrication of the Raman pumps. Moreover, this solution affords twice the number of pump units for a given number of pump channels. This doubles the overall cost and the costs of the PBC are high.
Another alternative is to use a single semiconductor pump laser for each required pump wavelength and to depolarize it by use of a polarisation maintaining fibre of a length L from about 10 to 100 m where its principal axis is rotated by 45° with respect to the principal axis of the polarisation maintaining fibre pigtail of the laser. This principle was used in Bigo et al. ECOC'00 (postdeadline paper no. 2), Munich 2000. In the experiment described in this paper each of the four pump lasers used for distributed Raman amplification was depolarized by this technique. However, the solution is not fully satisfactory, as firstly, a perfect depolarisation could not be obtained and secondly, the introduction of the polarisation maintaining fibre induced losses in the pump path of about 0.5 dB.
In addition all prior art systems described in the foregoing do not reduce the pump cross-talk.
The underlying problem of the present invention is therefore to avoid polarization effects in multichannel WDM pumps used in Fibre Raman Amplifiers (FRA) for localized as well as distributed Raman amplification while maintaining a high pumping efficiency and to reduce cross-talk between pump wavelengths.